Cat pees on the carpet! Help!?

When we saved her as a 6-7 week old lost kitten, I decided to feed her dry food with a little bit of canned food in the morning and at night as a treat for her. I also provided the dry food so she could have it whenever she wanted. She was so tiny I wanted her to gain weight. Now she is a grown 6 month old cat and I have lost my job and can't afford the canned food anymore, but she expects it and if she doesn't get it, she will go and defecate all over the house. This last time she actually peed on my light cream carpet, and I can't stand that. I will put up with a lot but not urine on my carpet, where me and my children sit and play. What kind of experiences have you had with your cats and is there a hope of retraining her, and how? I can't stand the thought that someone whom we love so much is now making me feel terrorized by not knowing of where and what kind of damage she will cause next. All suggestions welcome!